This site is for people who like plants -- growers, enthusiasts, aesthetes, novices and professionals, those who appreciate wild things and those who appreciate the cultivated. I garden in Chelsea, and I've been visiting people's yards for 20+ years in the course of my work. My goal is to make this blog a community project, so if you share my interests, please consider becoming a participant and contributing content -- Guerin. Info: greenstreet@mindspring.com

Thursday, May 3, 2012

Big bad diseases on landscape conifers

After years of chasing after the familiar Diplodia tip-blight on Austrian pines, we have found that another fungal pathogen has proliferated and spread to become the number one killer of this popular landscape conifer. This 'newer' and less-familiar disease is Dothistroma needle-blight. The photo shows the usual damage -- the needles turn brown from the tip, darker brown bands show up on infected needles, and a small amount of healthy green tissue remains at the base of the needles. Often the entire needles will eventally turn brown. The expanding bud will produce new needles but these will become infected soon after emergence. The result is branch die-back and sometimes tree mortality. We at GreenStreet (my tree service) don't have enough experience with it to be able to predict the effectiveness of chemical controls. Whereas fungicides are applied in April and May to control the old tip-blight disease, the life-cycle of Dothistroma is different and plant pathologists are recommending sprays in June and July. Austrian pine has proven to be a very useful landscape conifer because of its tolerance of clay soils. However, you might do best to just stay away from it for now because of disease pressures.

Another plant disease of note is cedar-apple rust. On native old-field junipers, the fungus produces swollen nodules that drip with slimy orange 'worms.' This occurs during the first rains of spring after the plants come out of dormancy (i.e., this past week). Yucky beautiful. The spores created by these structures go on to infect the leaves of hawthorns. Kudos to whomever figured out the crazy life-cycle of the rust fungi. The organism is mostly harmless.

I suppose something should be said about spruce trees which are declining in great numbers throughout our area. The situation is a complicated mess. A number of fungal agents appear to be involved, and questions remain as to which are causal agents and which are incidental. Until a few days ago, we were happy to lump them all together as 'needlecast' diseases, in hopes that a few judiciously timed sprays each year would keep the problems at bay. Thus it was disconcerting to read the following headline in a recent bulletin from Michigan State University Extension: "Michigan awash with Phomopsis cankers on spruce trees." The bulletin continues to state that "we now know that a group of Phomopsis strains of unknown species are at the center of the current landscape spruce problems . . and is now causing mature tree defoliation, branch death and, in some cases, tree death." Making things more difficult is the fact that symptoms appear much like needlecast, and it is necessary to skin off a lot of thin bark to determine the presence of the canker. The final paragraph begins "it appears we are in a cloud of Phomopsis spores, the likes of which we have never seen before." Trying to treat spruces with fungicides is becoming a shot in the dark.